Laser induced thermal transfer of materials from a donor sheet to a receptor layer has been described in the patent and technical literature for nearly thirty years. However, few commercial systems have utilized this technology. Exposure fluences required to transfer materials to a receptor have been, at best, on the order of 0.1 Joule/cm.sup.2 (i.e., J/cm.sup.2). Consequently, lasers capable of emitting more than 5 Watts of power, typically water-cooled Nd:YAG lasers, have been required to produce large format images (A3 or larger) in reasonable times. These lasers are expensive and impractical for many applications. More recently, single-mode laser diodes and diode-pumped lasers producing 0.1-4 Watts in the near infrared region of the electromagnetic spectrum have become commercially available. Diode-pumped Nd:YAG lasers are good examples of this type of source. They are compact, efficient, and relatively inexpensive. In order to use these new sources in a single-beam, large format imaging system, the exposure fluence of thermal transfer materials should be reduced to less than 0.04 J/cm.sup.2 and the exposure pixel dwell time should be less than 300 nanoseconds. There have been many unsuccessful efforts in the art to achieve this goal.
Separately addressed laser diode arrays have been utilized to transfer dyes in color proofing systems. For example, U.S. Pat. No. 5,017,547 describes the binderless transfer of dye from a dye-binder donor sheet to a polymeric receptor sheet. In that process, dye molecules are vaporized or sublimed via laser heating. These molecules traverse the gap between the donor and receptor and recondense on the receiver. The donor and receptor are separated by spacer beads. This technique has several disadvantages. First, the state change of dye (i.e., solid to vapor) requires high energy fluences (.about.0.5 J/cm.sup.2) and relatively long pixel dwell times (.about.10 .mu.sec), thus requiring multiple beam arrays for rapid imaging of large format areas. A plastic-coated receptor is required for proper laser addressed transfer. The image on this receptor must then be retransferred to plain paper, a step that adds cost, complexity, and time to the printing process.
U.S. Pat. No. 3,978,247 discloses the use of binderless, abrasion-resistant dyes coated on transparent donors. The dyes employed have low vaporization temperatures and low heats of vaporization. The binderless coating contains less thermal mass and therefore, the exposure energy required to transfer the dye should be less than that required in the system of U.S. Pat. No. 5,017,547. However, no examples demonstrating less exposure energy were disclosed in U.S. Pat. No. 3,978,247. In general, however, dyes are inadequate for critical proofing applications where the proof is used as a diagnostic tool to evaluate press performance. Printing presses print with pigments; therefore, the proof should be printed with pigments to properly match the characteristics of the printing press. Laser addressed proofing with pigments is difficult because pigments typically have a high vaporization temperature and are intrinsically less volatile than dye molecules, making direct thermal transfer of pigments difficult.
Exothermic heat-producing reactions have been used for the thermal transfer of inks. For example, in U.S. Pat. No. 4,549,824 aromatic azido compounds were incorporated into thermal transfer inks. When heated to 170.degree. C., the aromatic azido compound melts the ink and allows it to flow into a receptor, such as plain paper. The heat generated by the decomposition of the aromatic azido compound reduces the amount of heat that must be supplied by the thermal head or laser source, thereby improving the overall imaging throughput. However, the process occurs over a relatively long time scale (.gtoreq.1 msec), thereby resulting in significant heat diffusion and heat loss. In addition, pressure between the donor and receptor is required to maintain uniform transfer. An optically transparent means of applying pressure (e.g., a cylindrical lens or a flat glass plate) is difficult to employ in high resolution laser-based imaging systems.
Laser induced propulsive transfer processes can be used to achieve exposure fluences and pixel dwell times that are substantially less than those of the previously disclosed processes. U.S. Pat. No. 3,787,210 discloses the use of laser induced propulsive transfer to create a positive and negative image on film. A clear substrate was coated with heat-absorbing particles dispersed in a self-oxidizing binder. In that patent, the heat absorber was carbon black and the binder was nitrocellulose. The donor sheet was held in intimate contact with a receptor. When the coating was locally heated with a laser, combustion in the binder was initiated, thus blowing the carbon black onto the receptor. The receptor could be paper, adhesive film, or other media. The self-oxidizing binder was employed to reduce the exposure fluence required to achieve imaging.
In U.S. Pat. No. 3,964,389, crosslinkable resins were added to a carbon black/nitrocellulose coating and the material was transferred to aluminum by imagewise heating with a laser. The resin was thermally crosslinked on the aluminum to produce a lithographic printing plate.
U.S. Pat. No. 3,962,513 discloses the use of a dual-layer coating construction for the production of lithographic printing plates. The first layer was a coating of carbon black and nitrocellulose binder coated on top of a clear substrate. An overlying layer of crosslinkable, ink-receptive resin was coated over this propellant layer. Upon laser heating, the resin was transferred to an aluminum plate. The run length and the image sharpness of the resulting plate were improved with this construction. That patent discusses the advantages of placing the propellant in a separate layer below the material to be transferred. Unlike the single layer case, the expansion of gas serves to propel the transfer material from behind, thus achieving forward propulsion of the transfer material toward the paper and minimal lateral dispersion. Exposure fluence measurements were not disclosed, but examples presented later herein show that more energy is required to expose media that use commercially available nitrocellulose in the underlying propellant layer than materials used in the present invention.
Nitrocellulose propellant layers have several undesirable characteristics when employed in imaging systems, as pointed out in British Patent Application No. 2,176,018. For example, mixed oxides of nitrogen are produced during decomposition of nitrocellulose, forming a corrosive acid that can damage the imaging apparatus. Nitrocellulose with high nitration levels is required to produce sufficient amounts of gas during imaging. However, this form of nitrocellulose presents safety and storage risks (explosion hazard). In addition, exact and uniform reproduction of nitrocellulose is difficult to achieve as explained in PCT Application No. 87/02904. As a consequence, an alternative to nitrocellulose as a blowing agent is needed.
U.S. Pat. No. 4,245,003 discloses the use of graphite in an ethyl cellulose binder for producing films. By using graphite, the imaged areas of the negative transparency were blown clean. In that case, the binder was not self-oxidizing. No exposure fluence information was disclosed. Graphite images are not useful in color proofing applications.
The use of decomposable polymers for color proofing applications is disclosed in PCT Application No. 90/01635. A single layer containing pigment, infrared dye, acid initiator, and polymeric binder was coated on a transparent substrate. Upon heating with a Nd:YAG laser, an acid was produced that rapidly decomposed the polymer to produce gas. An exposure energy of 0.1 J/cm.sup.2 was required to transfer this material. The gas serves to propel the pigment to a receiver such as plain paper, but that construction does not provide the flexibility to transfer many types of materials due to the compatibility of the components in the single layer construction. For this reason, the use of a two layer construction that does not require excessive exposure energy would be preferred.
PCT Application No. 91/06813 discloses methods and materials for thermal imaging using an "ablation-transfer" technique. The donor element for that imaging process comprises a support, an intermediate dynamic release layer, and an ablative carrier topcoat. The topcoat carries the colorant. The dynamic release layer may also contain infrared-absorbing (light to heat conversion) dyes or pigments. No specific examples of azido group-containing polymers were given. Nitrocellulose as a binder was disclosed.
Copending U.S. application Ser. No. 07/855,799 discloses ablative imaging elements comprising a substrate coated on a portion thereof with an energy sensitive layer comprising a glycidyl azide polymer in combination with a radiation absorber. Demonstrated imaging sources were infrared, visible, and ultraviolet lasers. Solid state lasers were disclosed as exposure sources although laser diodes were not specifically mentioned. That application concerns formation of relief printing plates and lithographic plates by ablation of the energy sensitive layer. No mention of utility for thermal mass transfer was made.
U.S. Pat. No. 5,089,372 discloses transfer recording media comprising a substrate having sequentially coated thereon a layer of a photolyzable compound (e.g., aromatic diazo and azide compounds) and a solid ink layer.
Japanese Kokai Patent Appln. No. 64-14081 discloses a thermal transfer recording medium comprises of an interlayer containing a photolyzable compound; light-to-heat conversion layer; and thermally transferable ink layer deposited on one side of a transparent support. Aromatic azide compounds are preferred for use as the photolyzable compound.
In view of the foregoing, what is needed in the industry are thermal transfer donor elements which overcome the above-disclosed deficiencies of conventional systems.